1. Field of the Invention
The present invention relates to an improvement in an ultrasonic transducer using a laminated piezo-electric element and more particularly to an ultrasonic transducer with improved directivity characteristics and improved transient characteristics (pulse characteristics).
2. Description of the Prior Art
Ultrasonic transducer for use in the air has been proposed and includes laminated piezo-electric ceramic elements which are designed to work at resonance point or anti-resonance point. Further, since the mechanical impedance of air is very smaller than that of the piezo-electric ceramic element, the laminated element is connected to a diaphragm for attaining mechanical impedance matching therebetween.
In video camera having automatic focussing mechanism for its objective lens by means of ultrasonic distance measurement, the measurement must be continuously made. Such continuous measurement requires a good transient characteristic in order to avoid error of measurement. For such good transient measurement, short rise up and falling down time are necessary. On the other hand, in such video camera uses zoom lens as objective lens, and distance measurement for such zoom lens must be made with a sharp directivity corresponding to narrowest picture angle of the zoom lens.
Hitherto, ceramic ultrasonic transducer is known as the apparatus of a high sensitivity, high durability against moisture or acidic or salty atmosphere and high S/N ratio due to its resonance characteristic. But the ceramic ultrasonic transducer has had bad transient characteristic due to its very high mechanical Q value.
A typical example of conventional ultrasonic transducer is shown in FIG. 1, which is a sectional elevation view along its axis. As shown in FIG. 1, a lower end of a coupling shaft 2 is fixed passing through a central portion of a laminated piezo-electric element 1 with the upper part secured to a diaphragm 3. The laminated piezo-electric element 1 such as a ceramic piezo-electric element is mounted at positions of nodes of oscillation via a flexible adhesive on tips of supports 4. Lead wires 9,9' of the laminated piezo-electric element is connected to terminals 6,6' secured to base 71 of a housing 7, which has a protection mesh 8 at the opening thereof.
FIG. 2 is a graph showing envelope of radiated ultrasonic wave transmitted when the transducer is supplied with the ultrasonic wave during the time of 0 to 2 m sec of time graduated on the abscissa. As is observed in FIG. 2, the response of the transducer, i.e., the rise time and fall time are relatively long, both being of the order of 2 m sec. When data signal is sent and received by use of such ultrasonic transducer, time density of the data, or data transmission speed is limited by such relatively long rise time and fall time. If a high density data signal is sent and received via such transducer, for example, in ultrasonic wave distance measurement, data become mixed with the tailing part of the preceding data. Accordingly accurate sending and receipt of data is not attained.
Furthermore, when it is intended to obtain a sharp directivity with such device as shown in FIG. 1, use of larger laminated piezo-electric element 1, larger diaphragm 3, and larger supports 4 must be made much large, and pure piston disc motion of such large diaphragm, if used, become hard to realize. Therefore, sharp directivity has been hard to realize. When, in order to attain a sharp directivity, a horn is intended to be combined to such apparatus with large components, then, improvement of the transient characteristic through lowering of the mechanical Q value of the ultrasonic vibration system becomes more difficult.